1. Field of the Invention
The present invention relates to an image sensing apparatus driving method, image sensing apparatus, and image capturing system.
2. Description of the Related Art
Digital cameras, video cameras, portable phones equipped with cameras are recently starting to use a CMOS image sensing apparatus. The indices of performance of an image sensing apparatus are the number of pixels and the image signal readout rate.
To increase the number of pixels, a method of making a plurality of pixels share an active element in a MOS image sensing apparatus has been proposed. In Japanese Patent Laid-Open No. 2005-198001, four pixels share one floating diffusion and one active element. This reduces the pixel pitch and increases the number of pixels per predetermined area.
On the other hand, to speed up pixel signal readout, the number of output channels is increased in a MOS image sensing apparatus. Alternatively, to speed up pixel signal readout, the number of pixels is compressed by thinning pixel signals in accordance with the application purpose, thereby increasing the frame rate.
To speed up pixel signal readout, the number of pixels may be reduced by adding the signals of a plurality of pixels and then outputting the pixel signals so that the frame rate increases. In Japanese Patent Laid-Open No. 9-46596, a plurality of pixels share one floating diffusion and one active element. The floating diffusion adds electric carriers acquired by a plurality of pixels so that the pixel signals in the column direction can be added without an increase in the readout time.
The readout method described in Japanese Patent Laid-Open No. 9-46596 cannot add pixel signals in the row direction. This may make it impossible to sufficiently compress an image. It may therefore be impossible to sufficiently improve the frame rate when the number of pixels increases.